barLevel: Use setters instead of methods

Switching to getters/setters make the code suitable for using
with Tweener and as GObject properties, both of which we'll
do soon enough.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/385

js/ui/barLevel.js

@@ -34,7 +34,11 @@ var BarLevel = class {
         this.connect('value-changed', this._valueChanged.bind(this));
     }
 
-    setValue(value) {
+    get value() {
+        return this._value;
+    }
+
+    set value(value) {
         if (isNaN(value))
             throw TypeError('The bar level value must be a number');
 
@@ -42,7 +46,13 @@ var BarLevel = class {
         this.actor.queue_repaint();
     }
 
-    setMaximumValue(value) {
+    // eslint-disable-next-line camelcase
+    get maximum_value() {
+        return this._maxValue;
+    }
+
+    // eslint-disable-next-line camelcase
+    set maximum_value(value) {
         if (isNaN(value))
             throw TypeError('The bar level max value must be a number');
 
@@ -51,7 +61,13 @@ var BarLevel = class {
         this.actor.queue_repaint();
     }
 
-    setOverdriveStart(value) {
+    // eslint-disable-next-line camelcase
+    get overdrive_start() {
+        return this._overdriveStart;
+    }
+
+    // eslint-disable-next-line camelcase
+    set overdrive_start(value) {
         if (isNaN(value))
             throw TypeError('The overdrive limit value must be a number');
         if (value > this._maxValue)
@@ -196,9 +212,5 @@ var BarLevel = class {
     _valueChanged() {
         this._customAccessible.notify("accessible-value");
     }
-
-    get value() {
-        return this._value;
-    }
 };
 Signals.addSignalMethods(BarLevel.prototype);